The present invention can be applied to a range of semiconductor devices, although it is especially effective in light-receiving elements such as photodetectors and solar cells. The background of the invention is described below with reference to solar cells as a specific example of the prior art.
Conventional terrestrial solar cells are generally made of thin wafers of silicon (Si) in which a rectifying or p-n junction has been created and electrode contacts, that are electrically conductive, have been subsequently formed on both sides of the wafer. A solar cell structure with a p-type silicon base has a positive electrode contact on the base or backside and a negative electrode contact on the n-type silicon or emitter that is the front-side or sun-illuminated side of the cell. The “emitter” is a layer of silicon that is doped in order to create the rectifying or p-n junction and is thin in comparison to the p-type silicon base. It is well-known that radiation of an appropriate wavelength incident on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in the semiconductor body. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions. The electrons move to the negative electrode contact, and the holes move to positive electrode contact, thereby giving rise to flow of an electric current that is capable of delivering power to an external circuit.
FIG. 1 is a process flow diagram, shown in side elevation, illustrating the fabrication of a semiconductor device according to conventional processes and materials.
In FIG. 1A, a p-type silicon substrate 10 is provided. The substrate may be composed of single-crystal silicon or of multicrystalline silicon. As shown in FIG. 1B, an n-type layer 20 in FIG. 1B, is formed to create a p-n junction. The method used to form the n-type layer is generally by the thermal diffusion of a donor dopant from Group V of the periodic table, preferably phosphorus (P), using phosphorus oxychloride (POCl3). The depth of the diffusion layer is generally about 0.3 to 0.5 micrometers (μm). The phosphorus doping causes the surface resistance of the silicon to be reduced to between several tens of ohms per square (Ω/□) to something less than 100 ohms per square (Ω/□). In the absence of any particular modification, the diffusion layer 20 is formed over the entire surface of the silicon substrate 10.
Next, one surface of this diffusion layer is protected with a resist or the like and the diffusion layer 20 is removed from all but one surface of the article of FIG. 1B by etching. The resist is removed, leaving the article of FIG. 1C.
Next, as shown in FIG. 1D, an insulating silicon nitride Si3N4 film, or a silicon nitride SiNx:H film is formed on the above-described n-type diffusion layer to form an anti-reflective coating (ARC). The thickness of the Si3N4 or SiNx:H anti-reflective coating 30 is about 700 to 900 Å. As an alternative to silicon nitride, silicon oxide may be used as an anti-reflection coating.
As shown in FIG. 1E a silver paste 50 for the front electrode is screen printed and then dried over the silicon nitride film 30. In addition, an aluminum paste 60 and a backside silver or silver/aluminum paste 70 are screen printed and successively dried on the backside of the substrate. Co-firing of front and backside pastes is then carried out in an infrared furnace at a temperature range of approximately 700° C. to 975° C. in air for a period of from several minutes to several tens of minutes.
As shown in FIG. 1F, aluminum diffuses from the aluminum paste into the silicon substrate 10 as a dopant during firing, forming a p+ layer 40 containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
Firing also converts the aluminum paste 60 to an aluminum back electrode 61. The backside silver or silver/aluminum paste 70 is fired at the same time, becoming a silver or silver/aluminum back electrode 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, thereby achieving electrical connection. The aluminum electrode also dopes the silicon to form a p+ layer 40. Because soldering to an aluminum electrode is impossible, a silver back tab electrode is formed over portions of the back side as an electrode for interconnecting solar cells by means of copper ribbon or the like.
During the co-firing, the front electrode-forming silver paste 50 sinters and penetrates through the silicon nitride film 30, and is thereby able to electrically contact the n-type layer 20. This type of process is generally called “fire through” or “etching” of the silicon nitride This fired through state is apparent in layer 51 of FIG. 1F.
Conventional front electrode silver pastes contain silver powder, an organic binder, a solvent, a glass frit and may contain various additives. The silver powder functions as the main electrode contact material and provides for low resistance. The glass frit may contain lead or other low melting point constituents to give a softening point of about 300 to 600° C. The glass frit also provides for adhesion of the sintered silver to the silicon. Additives may be used as additional dopants to modify the n-type conductivity. During firing, the glass melts and penetrates through the silicon nitride film so that the silver electrically contacts the n-type silicon layer. The interface structure after firing consists of multiple phases: substrate silicon; silver-silicon islands; silver precipitates within an insulating glass layer; and bulk sintered silver. As a result, the contact mechanism is a mix of ohmic contact by the silver-silicon islands and silver precipitates and tunneling through thin layers of the glass. The electrode contacts to the solar cell are important to the performance of the cell. A high resistance silicon/electrode contact interface will impede the transfer of current from the cell to the external electrodes and therefore, reduce efficiency. Compositions and firing profiles, therefore, of the conductive paste are optimized to maximize cell efficiency. However, the presence of glass at the metal-silicon interface inevitably results in a higher contact resistance than would be realized by a pure metal contact to silicon.
Difficulties associated with forming low resistance contacts to bipolar silicon devices exist. All elemental semiconductor contacts have a potential barrier that makes the contact rectifying. A Schottky barrier height (SBH) is the rectifying barrier for electrical conduction across a metal-semiconductor (MS) junction and, therefore, is of vital importance to the successful operation of any semiconductor device. The magnitude of the SBH reflects the mismatch in the energy position of the majority carrier band edge of the semiconductor and the metal Fermi level across the MS interface. At a metal/n-type semiconductor interface, the SBH is the difference between the conduction band minimum and the Fermi level. The lower the SBH, the better the contact to silicon. Low Schottky barrier height contacts to n-type silicon semiconductor devices are known. U.S. Pat. Nos. 3,381,182, 3,968,272 and 4,394,673, for example, disclose various silicides that form low SBH contacts to bipolar silicon devices when the metal is placed in contact with the silicon and heated. United State Patent Application No. 61/088,504 to Borland et al., discloses the use of metal nitrides as low Schottky barrier height contacts formed by reaction of silicon nitride with metals from Group 4B and 5B of the periodic table wherein some metal silicide formation may occur. Glass free pure silicide contacts to n-type silicon for front face electrode contacts to silicon solar cells are not disclosed.
Novel compositions and processes for forming front electrode contacts of photovoltaic devices are needed which provide superior reduction in contact resistance, maintain good adhesion and use safe, inexpensive and readily available materials and processes.